Instabilities in granular materials and application to landslides

Landslides or questions related to slope stability are usually considered in the framework of plastic limit analyses. Recent progress has made it possible to describe some failure modes in the framework of the theories of bifurcation of the strain mode by plastic strain localization and the shear-banding phenomenon. We propose in this paper to reconsider the question of landslide analysis by taking into account an appropriate instability criterion. As soils are strongly non-associated materials, unstable states can be reached strictly inside the plastic limit condition (which defines the set of admissible stresses). In the first part of this paper, we describe the constitutive model. Then Lyapunov's definition of stability allows us to detect unstable stress–strain states from experimental evidence. These unstable states are analysed by considering the sign of the second-order work. The stability analysis, performed for loose and dense sands under plane strain conditions, shows a large domain of instabilities in the stress space. This method is applied to some boundary-value problems by finite elements computations. It is shown finally by examples that such unstable stress–strain states are observed in our FEM modelling of slope problems. Copyright © 2000 John Wiley & Sons, Ltd.